A murderer is on the run after escaping from a prison in north London, the Metropolitan police have warned.

John Massey, 64, excaped from Pentonville prison in Islington at around 6.30pm on Wednesday. Scotland Yard said he is considered "potentially dangerous" and should not be approached.

Massey was sentenced to life imprisonment for the murder of a man at a pub in Hackney, east London in 1975.

It has not yet been revealed how Massey escaped from Pentonville, but an investigation into how he managed to elude staff is due to be launched by the the prison service.

A spokesperson said: "At approximately 6.30pm on Wednesday 27 June a prisoner was found to be missing from HMP Pentonville. Police were immediately informed and the matter will be the subject of a thorough investigation."

Scotland Yard have advised members of the public not to approach the offender and instead call 999 if he is spotted.

A police spokesman said: "Police are appealing for help in tracing a convicted murderer who has escaped from HMP Pentonville.

"He is considered potentially dangerous and should not be approached by the public if he is seen."

At 35 years and counting, Massey is one of Britain's longest-serving prisoners. He was released on parole from an open prison in Derbyshire in June 2007. But he breached the terms of his curfew, he says due to his father's imminent death. Massey turned himself in to the police and was immediately recalled to prison.

In May 2010, after he had been decategorised and sent to another open jail, Ford, in West Sussex, Massey walked out to stay with his sister Carole while she died in hospital. This time he did not hand himself in, instead staying with his mother until he was tracked down 10 months later and returned to jail.

Anyone with information of Massey's whereabouts is asked to call Islington police on 020 7421 0296, or to inform Crimestoppers anonymously on 0800 555 111.

Pentonville houses up to 1,250 category B and C male prisoners, rather than the most serious category A offenders.

In 2009 a convicted arsonist escaped from the same jail by clinging to the underside of a security van. Julien Chautard was sentenced to seven years in jail at Snaresbrook Crown Court in February 2009, but spent just minutes inside Pentonville before slipping out underneath the van he had arrived in.